In the typical nuclear magnetic resonance (NMR) analysis instrument, a sample is placed within a volume situated within a homogeneous region of magnetic field. Excitation and detection of resonance is obtained from a suitably placed coil (or coils) ordinarily spaced with respect to the sample and preferably enveloping it. Quite typically, for modern Fourier transform resonance spectroscopy, the sample is contained within a cylindrical tube disposed coaxial with, and within, a single coil and means are provided to rapidly rotate the sample tube about its axis to average any residual inhomogeneities for the magnetic field.
The material environment of the sample volume of typical prior art apparatus may contain a number of substances: the sample container, usually glass and possibly including a stopper delimiter of nylon or similar inert material; a conductive material forming the RF coil conductor, commonly copper, aluminum, silver, gold or platinum or a combination of these materials; a coil form supporting the coil; a bonding agent for securing the conductor to the coil form; one or more holes in the coil form for interconnection of coil winding components; and, air permeating all available spaces. These materials distinct from the sample and solvent itself, exhibit various magnetic susceptibilities and influence the signal by varying the magnetic field distribution throughout the sample. The relative rotation of the sample and the RF field acts to average sources of magnetic perturbations of non-cylindrical geometry whereby the average produces an equivalent cylindrical symmetry. Some of these sources have been considered in prior art compensatory schemes. Coil materials and bonding agent materials have been considered by Anderson, et al., U.S. Pat. No. 3,091,732 where it was sought to provide coil materials and bonding agents for securing the coil to a coil form, which materials were required to exhibit a magnetic susceptibility approximating air, (in which these components are necessarily submerged). The inhomogeneity due to structure present within the active volume of the spectrometer is compensated by fabricating a material which has magnetic properties identical with the properties of the solvent whereby axial homogeneity over the sample volume is contained. This is described in U.S. Ser. No. 482,344. The compensation of the geometric axial distribution of materials forming a saddle coil is discussed in U.S. Ser. No. 534,899.
In the prior art the form supporting the RF probe coil is a cylinder of glass or similar material. There are advantages for mounting the coil on the internal surface of the form to obtain closer coupling with the sample but there remains the requirement to provide access to the coil terminals. Holes may be provided in the form or else (saddle) coil leads must be directed along the form with insulation provided at the crossover point which necessarily occurs with saddle coil structure. Where the conductor is specially fabricated to exhibit a desired magnetic susceptibility the axial magnetic discontinuity as introduced by holes or insulating materials becomes relatively significant. Any such discontinuity within the active sample region and shorter than the axial extent of the active region will cause such a significant perturbation. For a spinning sample the discontinuity can be averaged azimuthally. If the discontinuity is lengthened in axial extent, the preferred cylindrical averaging will homogenize the perturbation and effectively eliminate it.